MKirchhoff
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# Posted: 5 Nov 2021 02:15am
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You sure don't want to insulate with fiberglass before the cabin/floor is dried in. Putting insulation between the joists, and then covering the bottom of the joists with plywood or OSB requires working from underneath, which isn't always so easy (as in try laying on your back, holding a piece of 4x8 ply against the bottom of the joists while you screw or nail up). There should not be any air space between the floor and the insulation beneath. Air, alone, is a poor insulator. In my experience with batt insulation, anything nailed or stapled to the bottom of the joists does not keep the insulation tight to the subfloor.
I find it easier to work from above. Following is a method used by log builders in Alaska, endorsed by the Alaska Housing Finance Corporation (Alaska Log Building Construction Guide, 1999). I've improved on it slightly, I think.
After joists are in place, nail a layer of OSB on top, marking joist locations with a chalk line. Then lay XPS foam (the blue or pink stuff) in 2'x8' x 1.5" panels at right angles to the joists, separating each run with a 2x2 screwed into the joist crossings. A second layer of foam insulation is added perpendicular to the first, again separated by 2x2s, screwed at each wood-to-wood crossing. Keep a rim of 2x4(s) or 2x6(s) on the flat all around the platform perimeter, flush with the top surface of insulation. This wood will bear the weight of the walls/logs down to the joists/rim below. Finally nail your 3/4" T&G plywood on top of this assembly, screwing down into the 2x2s, and into the 2x lumber around the perimeter.
There is no measuring and cutting to fit insulation to joist spaces. You've nailed and screwed down instead of from your back, reaching up. Gravity works to hold materials down, and tight, as you build. You have eliminated thermal bridging through the joists. The top T&G ply is fully supported by high-density foam and 2x2s. Because it's T&G, there is no need to worry about the dog's water dish tipping or mop water reaching the insulation (you can add a bead of caulk to the T&G seam when installing, if you wish). If it rains during the build, that's OK. The insulation will not be compromised, and any moisture will dry outward.
Yes, the platform deck is raised up a few inches, but the platform can "start" anywhere. There just has to be solid wood support from heavy wall logs (or wall framing) down to the joists spanning the beams.
As others have noted, closing in the space beneath the platform with any kind of material between the perimeter piers reduces airflow, and will make the floor warmer. Between that, and 3 inches of XPS rigid foam, you'll have yourself a warmer floor, and one that was quite a bit easier and faster to build.
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